Glycolysis is the first major pathway cells use to harvest energy from glucose. It happens in the cytoplasm and does not require oxygen, so it can operate in both aerobic and anaerobic conditions. The pathway matters because it provides quick ATP and supplies molecules that feed into later stages of cellular respiration.
In one pass, a 6-carbon glucose molecule is split into two 3-carbon pyruvate molecules.
Glycolysis has an energy investment phase and an energy payoff phase. The cell first spends 2 ATP to make glucose more reactive, then later produces 4 ATP and 2 NADH as the pathway continues. The net gain is 2 ATP, 2 NADH, and 2 pyruvate per glucose.
If oxygen is available, pyruvate can enter the mitochondrion for further oxidation, but if oxygen is limited, pyruvate can be converted by fermentation to regenerate NAD+.
Key Facts
- Overall glycolysis: glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O
- Net ATP yield per glucose = 4 ATP produced - 2 ATP used = 2 ATP
- Each glucose molecule produces 2 pyruvate molecules, and each pyruvate has 3 carbons.
- Glycolysis occurs in the cytoplasm, not inside the mitochondrion.
- NAD+ is reduced to NADH when it accepts high-energy electrons during glycolysis.
- Glycolysis does not directly require O2, but aerobic respiration needs O2 later to keep NAD+ available.
Vocabulary
- Glycolysis
- Glycolysis is a series of enzyme-controlled reactions that breaks one glucose molecule into two pyruvate molecules.
- Pyruvate
- Pyruvate is a 3-carbon molecule formed at the end of glycolysis that can enter aerobic respiration or fermentation.
- ATP
- ATP is a molecule that stores usable chemical energy for many cellular processes.
- NADH
- NADH is an electron carrier that stores high-energy electrons collected during glucose breakdown.
- Cytoplasm
- The cytoplasm is the fluid-filled region of the cell where glycolysis takes place.
Common Mistakes to Avoid
- Saying glycolysis produces a net of 4 ATP is wrong because 2 ATP are used in the investment phase, so the net gain is only 2 ATP per glucose.
- Placing glycolysis in the mitochondrion is wrong because glycolysis occurs in the cytoplasm before pyruvate enters the mitochondrion in aerobic cells.
- Thinking oxygen is required for glycolysis is wrong because glycolysis itself can run without oxygen, as long as NAD+ is regenerated.
- Forgetting that one glucose makes two pyruvate is wrong because the 6-carbon glucose molecule is split into two 3-carbon products.
Practice Questions
- 1 A cell breaks down 6 glucose molecules by glycolysis. What are the net numbers of ATP, NADH, and pyruvate produced?
- 2 During glycolysis, 2 ATP are used and 4 ATP are produced for each glucose. If 15 glucose molecules go through glycolysis, what is the net ATP gain?
- 3 A muscle cell has little oxygen available during intense exercise. Explain why glycolysis can continue for a short time and why the cell must regenerate NAD+.